Strongly basic anion-exchanging molded bodies and a method of manufacturing the same

ABSTRACT

Anion-exchanging molded bodies and a method of manufacturing the same. The aim is to find anion-exchangers which can be produced in any shape simply, cheaply and reproducibly, without the use of carcinogenic chloromethyl ethers. It has been established that halogenated polyethers, preferably epichlorhydrin polymers, can be treated with tertiary amines together with inert polymers to produce such anion-exchanging molded bodies by a phase-inversion process or evaporation of the solvent. These molded bodies can be in the form of blocks, balls or films.

FIELD OF THE INVENTION

The invention refers to new strongly alkaline ion exchanging moldedbodies and a method of manufacturing the same. These anion exchangersconsist of a polymer system which allows due to its special possibilityof processing and its mechanic properties to produce fibers, mats,compact bodies, balls and so on.

Ion exchangers are used to exchange ions in solutions. Furthermore theyare used in electrophoresis and ion chromatography as supportingmaterial. As foils they are purchased as ion exchange membranes.

DESCRIPTION OF THE PRIOR ART

To date, anion exchangers are mainly made on the basis of copolymers ofstyrene.

Apart from styrene, polysulfones, polyether sulfones or poly(vinylpyridine) derivatives can also be used as a matrix polymer. Thesematerials have the drawback that their design as thin flexible layers orcompact bodies is difficult. In addition, very toxic chloromethyl-methylethers must mostly be used to produce these polymers.

Moreover there are anion exchangers (e.g. DD 301 541) containingalkylidene epoxides bound on the basis of polyvinyl alcohol, whichresult from the reaction of epichlorhydrin with secondary or tertiaryamines. These anion exchangers can only be produced as balls.Furthermore the chemical stability against acids and bases isunsatisfactory because of the contents of the polyhydroxylated polymers.

Therefore the use of polyepichlorhydrin as base polymer is veryadvantageous because a highly chemical stability can be expected. Asurvey of the state of the art can be found e.g. in Advances in PolymerScience 70 (1985), Key Polymers p. 92 f. Using this base polymer systemthe problem arises that only weakly basic anion exchangers can bemanufactured. This problem is also clearly visible in the production ofsimilarly built polyelectrolytes from polyepichlorhydrin and tertiaryamines where the reaction is limited on oligomeric polyepichlorhydrin.

From DE 4328226, there are known anion exchange membranes on the basisof products from the reaction of polyepichlorhydrin and tertiary amines.These membranes, however, can only be produced as thick and compactfilms. It's impossible to produce shaped bodies or very thin films usedas separation layers.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a generally usable stronglyalkaline basic anion exchanger

1. which can be produced in different physical forms (e.g. block form;as film on supporting materials; as adhesive).

2. is strongly alkaline without the use of any toxic chloromethylatingsubstance.

3. is chemically stable (especially against high and low pH-values andagainst oxidation agents)

4. whose mechanical stability is satisfactory, and

5. is easy and inexpensive to produce.

It was found that it is possible to transform a special solution into asolid anion exchanger through different procedures, so that a lot ofsolid bodies can be produced. This solution contains reaction productsof polyepichlorhydrin with tertiary amines, especially1,4-diazabicyclo-(2,2,2)-octane in dipolar aprotic solvents, ifnecessary with suitable inert polymers and/or softeners.

The use of the components of this invention thus surprisingly solves thewell known problems for the production of solid bodies out of ionexchange polymers, as the polymer system, according to the invention,can be manufactured by different methods.

At first there is the possibility to fill a casting mold with thesolution and to evaporate the solvent afterwards. This method restrictsitself to the production of large size flat bodies e.g. mats or fibers,which have no sticking up contours from the surface. It can be extendedon bodies with a structure on the surface, if softeners are added to thesolution which fill the volume of the later incorporated water. Toproduce compact thick bodies the solution can be made solid by means ofan according coagulant. The best method to do that is to fill thesolution in a casting mold and to overlay it with the coagulant. Duringa certain time the coagulant diffuses in the body and makes it solid. Itis possible to add a crosslinking agent to the coagulant, e.g. primaryamines, or the active polymer is crosslinked by heat in a further step.

Finally the casting solution can be sprayed as a very thin film on asupporting material so that the ion exchanger can serve as asemipermeable layer. Through these shaping possibilities there is a widerange of use: the material can especially be used as support in thegel-electrophoresis, as diaphragm in fuel cells, in the form of nappedmats, as ion-conducting spacer for the electrodialysis, as a separatinglayer for gas and liquids and as an ion conducting adhesive. In additionthere has been found that the above mentioned reacting products possesquite new separating properties and therefore can be used as separatingmaterial in the gas separation and pervaporation.

DESCRIPTION OF THE INVENTION

The nature of the ion exchangers of the present invention is determinedby the possibilities of shape giving, which arise from the combinationof the reactive halogen containing polyether and the inert polymer withthe amine component. The reactive component has the general formula

    -- CR.sub.1 R.sub.2 --CR.sub.3 R.sub.4 --O!.sub.n --

wherein at least one of the residues R₁ -R₄ is a CH₂ Z group withZ=halogen. These are polymers of epihalohydrines, of1,4-dichlor-2,3-epoxibutane or copolymers of the both or co- orterpolymers together with ethylene oxide. Because of the technicalavailability of high molecular polyepichlorhydrin respective copolymersof epichlorhydrin and ethylene oxide respective terpolymers ofepichlorhydrin, they are used favorable. Epibromhydrin and epiiodhydrincan also be used although they are very expensive. Furthermore polymers,e.g. polyvinyl alcohol, can be used on which active polymers of the saidform are grafted.

The reactive component according to the present invention meetsdifferent fundamental preconditions: It is industrially easily availableand the production does not require chloromethylation as process step.It is inexpensive, of pure aliphatic structure so that materialsresulting therefrom do not have a tendency for pollution, and can beused with such inert polymers that are unsuitable for use with otherbasic polymers. Furthermore, the halogenated polymers according to thepresent invention have a significantly smaller molar mass of the monomerunit compared with chloromethylated polystyrenes or polysulfones. Theattainable fixed ion concentrations in the materials are significantlyhigher because there are more reactive groups per gram polymer.

Beside these favorable aspects, the main further difference of the usedreactive polymer compared to other reactive polymers is its lowerreactivity towards tertiary amines. Tertiary amines react with organichalogen compounds to a chemical uniform quaternary ammonium salt.Diamines can react with both amino groups at different molecules so thatcrosslinking and formation of the quaternary ammonium ions is possibleat the same time, if a polymeric halogen compound is used. It istherefore advantageous without any limitation of the invention to usetertiary diamines for quaternization and simultaneous crosslinking ofthe polymers used according to the invention. The physical properties ofthe amines must also be adjusted to the process. To produce a membraneeasily, it has to be possible to bound both the amine and the polyethertogether in the casting solution. Trimethylamine is a gas at standardconditions and thus can not be used in such a process. The choice of theaccording amine and perhaps of the inert polymer gains therefore akey-function as--through the above mentioned causes--the amine has adecisive influence on the suitable processing of the solution.

When using polymers according to the present invention, a certainreaction time must be taken into consideration for the quaternization ofthe amine in order to ensure that the amine does not evaporate with thesolvent used. This is attained in a simple manner by limiting the steamvolume over the film during evaporation or by adding a certain excess ofamine. The use according to the invention of1,4-diazabicyclo-(2,2,2)-octane as amine component is particularlyadvantageous because the chemical and physical limiting conditions aremet in an ideal manner and the substance is significantly activated forthe quaternization of the first nitrogen atom. On the other hand, theabove mentioned steps are essential during the reaction and evaporationof the solvent especially when using this amine for receiving good ionexchangers because this amine has a strong tendency to evaporate withthe solvent, especially DMF. It has also been found advantageous topre-react the casting solution at elevated temperature, mainly 80-120°C. This results in an approximately 10 to 20% pre-substitution, withoutencountering a crosslinking of the polymer in the solution. It ispossible, however, to carry out the substitution almost completely inadvance by protecting in the di- or polyamine each amino group thatexceeds one or by adding the amine in a appropriate excess. Furthermorea possibly crosslinked material which has a relatively high degree ofpresubstitution can be liquefied again with a ultraturrax mixer. Thus itis attained that the active polymer reacts with he amine component. Thisis especially important if it is desired to produce a body by phaseinversion in combination with the use of an inert polymer or to get afilm on a supporting material which is sensitive to temperature.

This polymer changed in such a manner represents a substance of its own.From a substitution degree of about 10% this polymer is soluble inwater. Through an excess of the amine component, a non crosslinked,soluble material can be produced which is substituted up to 60% and canbe precipitated and purified with acetone. The polymer contains twocomponents in a statistical distribution and can schematically bedesigned as follows:

    -- CH.sub.2 --CH(CH.sub.2 CI)--O!.sub.n -- CH.sub.2 --CH(CH.sub.2 N.sup.+ (C.sub.2 H.sub.4).sub.3 N)--O!.sub.m --

It is bifunctional as it contains besides the original chloromethylenegroups quaternary ammonium ions which can react both intramolecularlyand intermolecularly over the tertiary amine functions contained onthem. The ratio of both components and thereby of the indices n and mcan, as shown above, be adjusted so that polymers result with differentlinking abilities. In the case of small m values the polymerrespectively its solutions are crosslinking slowly at ambienttemperature. Therefore they must quickly be processed. It isadvantageous that these products processed to a film are crosslinkingand so keep their form. In a further kind of process the active polymercan be reacted so far, that it does not contain any longer remarkablyreactive halogen atoms. This polymer can be isolated and does not tendto crosslinking. Crosslinking only begins after mixing with more activepolymer not yet reacted. So these two components can be mixed, thesolution can be brought to the suitable form where the crosslinkingreaction takes place. It is favorable to heat the mixture. Generally atemperature of 80° is sufficient to obtain a reaction within a fewhours.

The so reached reaction product dissolved e.g. in methanol can be usedto process thin layers of these materials. In these cases the use ofmicroporous supporting materials was very advantageous to produce verythin films. These films are very hydrophilic, therefore they swell inwater. Consequently they can be used to separate water in organicsolvents by means of pervaporation. Surprisingly it was found that thesefilms can be used for the separation of acidic gases as CO₂ or H₂S--especially if the films are changed into the carbonate or sulfideform. Hereby the very favorable manufacturing ability allows to producevery thin layers.

The isolated, not crosslinked reaction product can be used as ionconductible adhesives. In an actually known manner bipolar membranes canbe produced by bonding cation--and anion exchanging membranes, as it isdescribed e.g. in DE-OS-3508206. In this case, however, there is theadvantage that the polymer is strongly basic and is charged in thisbipolar membrane and, thus, takes part in the ion transport. Moreoverthe adhesive is alkaline stable. This is necessary to obtain a durablepasting.

If an isolated not crosslinked reaction product of polyepichlorhydrinwith DABCO is heated, a solid, in water swelling ion exchanger with highfixed ion concentration is obtained. The ion exchanger shows a highalkaline stability and low tendency of oxidation. Due to its lowswelling it is especially suitable to standard tasks as the desalinationof aqueous solutions. Furthermore an affinity towards sulfate anions canbe observed. If this selectivity is not desired, the use ofpoly-epichlorhydrin-co-ethylenoxide as active polymer is recommended.Anion exchangers on this base material show a much lower affinity towardsulfate.

By adding an inert polymer, the properties of the resulting membranescan be further varied. The use of polyepichlorhydrin or copolymersaccording to the invention affords the additional possibility to bringinert polymers in the membrane, which could not be used before. Thus, afurther variation of membranes is attained because the mechanicalproperties are essentially determined by the inert component. Useful asinert polymers are those which are soluble in DMF, N-methylpyrrolidoneor cyclohexanone such as polysulfone, polyether sulfone, polyvinylidenefluoride, polyvinyl chloride, polymethacryl nitril, polyacryl nitril(PAN) or copolymers of the respective monomers.

It has surprisingly been found that--especially with newly producedpolysulfone solutions mixtures can be obtained very well suitable toproducing microporous bodies by casting the mixture in the desired formand submerging the form after this. A layer of precipitated polymer isproduced at the border to the polymer solution. This layer becomesthicker during several hours or--depending on the deepness of theform--some days and finally fills the form totally. This well knownphase inversion leads to a compact solid body swelling in water. Notonly thin films can be produced but also solid bodies till a deepness of3 cm which can be used in the electrophoreses as compact electrolytes oras a solid with a not even profile. In order to be used as a gel body inthe electrophoresis it is of great advantage to usepoly-(epichlorhydrin-co-ethylenoxid) as active polymer, as this polymeris well suited for the permeation of large anions because of its greatintramolecular distances of the fixed ions.

A special possibility of use results from producing burled mats whichcan be used as spacers in the electrodialysis. Because of theirconductibility they possess the property to decrease quite remarkablythe electrolytic resistance of the "chamber" and to enable thus a widedesalination. These mats can be optimized by variation of the castingmold so that they satisfy the necessary current conditions in the cell.

These mats can also be produced through evaporation of the castingsolution, if a softener is added to the casting solution. The softenerhas the function to anticipate the swelling degree of the mat. It mustnot evaporate at the given evaporating conditions and must dissolve inthe swelling medium (e.g. water) if the mat is swollen. Thus during theswelling the softener is dissolved and replaced by water. The matremains unchanged in its form so that profile shaped bodies can be takenout of the casting mold. Therefore all components which are soluble inwater and nonvolatile can be used as softeners, especially polyvinylalcohol, polyethylene glycol and his ethers, polyvinyl pyrrolidone andpolyvinyl amide. The necessary additional quantity of a softener has tobe fixed experimentally for every casting solution. The same effect canbe reached by evaporating only a part of the solvent, which means onlyas far as the made body does not swell in water.

With PAN, mixtures with polyepichlorhydrin can be made in every ratio.The addition of further primary, secondary or tertiary amines can be ofgreat advantage to increase the fixed ion exchange capacity orto--increase the crosslinking.

Furthermore, films can be obtained by casting or spraying of a solution.If this is processed on a reinforcement such as a asymmetricultrafiltration membrane, these films can be made very thin. By thisway, materials for selective separation layers can be obtained, whichcan be used e.g. in pervaporation, gas separation, reverse osmosis,dialysis, piezodialysis and so on. At this point, the advantageouslypossibility of addition of softener in the selective separation materialis given. By this way, the profile of properties can be variedfurthermore.

A further way of executing the invention are thin mattings, which arecoated with thin metal layers on both sides. These mats are produced asdescribed above. Their surface can be flat or provided with a profile.For the application of the metal layer, the ionic conductance of thematerial can be used by e.g. processing as described inDenki-Kagaku-oyobi-Kogyo-Butsuri-Kagaku, vol. 62(5) 1994 p. 425-433.Such films can be used e.g. as diaphragms in fuel cells. By coating e.g.Nickel, a system can be made, Which allows the electrochemical oxidationof carbon containing fuels, such as methanol. In such an anionexchanging system, the main advantage over cation exchanging systems isgiven by the possibility of using cheap nickel instead of platinum.

Advantage of these mattings as presented in the invention over commonion exchange membranes is the alkaline stability, which is necessary,because in alkaline fuel cells the transported ions are hydroxide ionsand the possibility of evolving a rough surface profile.

BEST MODE OF EXECUTING THE INVENTION

Taking into account the preceding description and without limitation ofthe invention, the preferred method is executed by adding to a solute ofactive polymer the amine component, preferably1,4-diazabicyclo-(2,2,2)-octane as substance or solution, at a massratio of reactive polymer to amine of 1:1-2.5, then a solution of inertpolymer is added, and subsequently the solution is heated to 50-100° C.until the viscosity of the solution rises considerably. The optimumtemperatures and time periods for this step depend on the respectivecomposition and concentration and must be determined experimentally eachtime. Typical values are however 70-90° C. and 2-12 h. Also the optimumratio of reactive polymer to amine should be adjusted experimentally.Typical concentrations of used polymer solutions are 10 to 15%.Subsequently, further amine components or further active polymer can beadded. The casting solution prepared by this method is the startingproduct for the producing of the molded bodies.

The preparation of anion exchangers in form of spheres or powder is bestdone without addition of inert polymer. The solution of reactive polymeris mixed with the amine, this solution is heated as described aboveuntil shortly before the point of gelation and this solution is stirredin a known manner in a non-solvent e.g. cyclohexane or paraffin oil withheating. The droplets formed by stirring are hardening by time and canbe isolated as ion exchangers.

EXAMPLES

1. A solution A is prepared by dissolution of 20 g polyepichlorhydrin in80 g DMF. A solution B is prepared by dissolution of 12 g PAN in 88 gDMF. A solution C is prepared by dissolution of 12.25 g1,4-diazabicyclo-(2,2,2)-octane in 88 g DMF.

1 ml solution A, 5 ml solution B and 1.2 ml solution C are mixed bystirring with a magnetic stirrer and heated to about 80° C. for 30 min.

This solution is cast in a beaker glass to a height of about 10 mm. Thisis treated some days with wet room air; up to this point, the whole masshas reacted to a yellow cloudy block. This is washed with water until itis free of DMF. It possesses a conductivity of 2 mS/cm.

2. A solution is prepared by mixing 100 ml solution A and 200 mlsolution C. This is heated on a oil bath to 100° C. The solution turnsfirst viscous, begins to gelatinize and turns cloudy after some time.The reaction product separates. Afterwards the solution is cooled, themass isolated and washed with water. It is helpful to swell the massdirectly in the vessel with water. An anion exchanger is obtained whichcan be produced in any size. It shows an ion exchange capacity of about6 meq/g polymer.

3. A solution A is prepared by dissolution of 10 g polyepichlorhydrin in90 g DMF. 12.3 g 1,4-diazabicyclo-(2,2,2)-octane are added, stirred andheated to 70° C. until the solution is noticeably more viscous (about 10h). This solution is cast in acetone and decanted from the precipitatedpolymer. This is washed several times with acetone and finally dissolvedin methanol. This solution is cast as a thin film on a ultrafiltrationmembrane and the methanol is evaporated. Afterward, this compositemembrane is heated in a drying funnel at 75° C. for 3 h.

This membrane shows a separating factor alpha (CH₄ /O₂)=1.5 with a fluxof 0.37 ml/(s bar cm²) CH₄. A separating factor alpha (N₂ /O₂)=1.6 isfound.

4. A casting solution as described in example 1 is cast on apolypropylene plate, which has a hexagonal pattern of 1 mm deep holeswith a diameter of 3 mm in 6 mm distances. The solution is coated with alayer of water until the produced matting is coagulated. It is isolatedafterwards and used as a spacer in a electrodialytic stack.

In a conventional electrodialysis device, such as described e.g. inChemie-Ing.-Techn. 47 (1975) p. 914 ff, this matting is used instead ofthe normally used spacer. As concentrate, a 1 N sodium carbonatesolution and as diluat, a 0.05 N sodium carbonate solution is used. Theconcentrate is circulated as known from a normal electrodialysis, thediluat outlet is fitted with a thrush and the product is collected andthe conductivity examined.

Applying a current density of 0.17 kA m⁻² results in a voltage drop of18 V per repeating unit. The product contains 54 mg/I sodium carbonateat a flux of 480 m⁻² h⁻¹.

5. An electrodialysis test as described in example 4 is made with thedifference residing in the substitution of the ion conducting spacer bya spacer made of polyester.

Applying a current density of 0.17 kA m⁻² results in a voltage drop of24 V per repeating unit. The product contains 108 mg/I sodium carbonateat a flux of 420 m⁻² h⁻¹.

6. A matting, as produced in example 4, is clamped between two chambersseparated by this membrane. The first chamber is filled with a 0.1 Nsolution of NiCl₂ the other is filled with a 0.1 N solution of NaBH₄. Atthe side of NiCl₂ a gray precipitation of nickel occurs. This procedureis repeated after turning the matting. The result is a matting coatedwith nickel on both sides.

A platinum mesh is installed as an electrode contact on both sides andis inserted in an apparatus, in which the electrode-ionexchanger-composite can be contacted with a 0.1 N NaBH₄ solution. Theother electrode works as oxygen cathode driven by air. A mixed potentialof 0.72 V is measured between the electrodes.

7. The solution of reacted polymer in methanol as described in example 3is cast on a cation exchange membrane (type: Neosepta CMX). On thissolution, a anion exchange membrane (type: Neosepta AMX) is prepared byrolling. This arrangement is installed in an electrodialysis apparatusas described in DE-OS 3508206 and a current is applied.

Both membranes stick together by the applied current. Applying a currentdensity of 1 kA m⁻², the resulting bipolar membrane shows a voltage dropof 1.3 V.

I claim:
 1. Anion exchanging molded bodies produced from a casting solution ofpolymers containing reactive halogen-containing functional groups; and tertiary aminesby evaporation of the solvent or phase inversion wherein the polymer which contains reactive halogen-containing functional groups is an epichlorohydrin polymer selected from the group consisting of polyepichlorohydrin, epichlorohydrin rubber, epichlorohydrin-ethylene oxide copolymers or terpolymers of epichlorhydrin and the tertiary amine contains at least two tertiary amino groups.
 2. Anion exchanging bodies according to claim 1, wherein an inert polymer selected from the group consisting of polysulfone, polyether sulfone, polyvinylidene fluoride, polyvinyl chloride, polymethacryl nitrile, polyacrylnitrile and copolymers of the respective monomers is added to the casting solution.
 3. Anion exchanger according to claim 1 wherein the tertiary amine is 1,4-diazabicyclo-(2,2,2)-octane.
 4. Anion exchanger according to claim 1, wherein a softener selected from the group consisting of polyvinyl alcohol, polyethylene glycol and its ethers, polyvinyl pyrrolidone and polyvinyl amid is added to the casting solution, which has been washed out after a form giving step.
 5. Anion exchanging molded bodies produced from a casting solution ofpolymers containing reactive halogen-containing functional groups; and tertiary aminesby evaporation of the solvent or phase inversion wherein the polymer which contains reactive halogen-containing functional groups is an epichlorohydrin polymer selected from the group consisting of polyepichlorohydrin, epichlorohydrin rubber, epichlorohydrin-ethylene oxide copolymers or terpolymers of epichlorhydrin, and wherein the tertiary amine contains at least two tertiary amine groups, and a softener selected from the group consisting of polyvinylalcohol, polyethyleneglycol and its ethers, polyvinylpyrrolidone and polyvinylamide that was added to the casting solution and subsequently has been washed out after casting.
 6. A method for making an anion exchanger according to claim 1 wherein the casting solution is prepared by;a) reacting said polymer containing reactive halogen containing functional groups with said tertiary amines, in a ratio of 1 part reactive polymer to 1-2.5 parts of tertiary amines b) increasing the viscosity by heating the casting solution to 50-100° C. c) adding a softener selected from the group consisting of polyvinyl alcohol, polyethylene glycol and its ethers, polyvinyl pyrrolidone and polyvinyl amide, d) pouring the casting solution into form e) removing said softener.
 7. Diaphragm for a fuel cell comprising anion exchanging molded bodies of the type produced from a casting solution ofpolymers containing reactive halogen-containing functional groups; and tertiary aminesby evaporation of the solvent or phase inversion wherein the polymer which contains reactive halogen-containing functional groups is an epichlorohydrin polymer selected from the group consisting of polyepichlorohydrin, epichlorohydrin rubber, epichlorohydrin-ethylene oxide copolymers or terpolymers of epichlorhydrin and the tertiary amine contains at least two tertiary amino groups.
 8. Selective layer of a membrane of the type selected from the group consisting of a gas separation membrane and a pervaporation membrane comprising anion exchanging molded bodies of the type produced from a casting solution ofpolymers containing reactive halogen-containing functional groups; and tertiary aminesby evaporation of the solvent or phase inversion wherein the polymer which contains reactive halogen-containing functional groups is an epichlorohydrin polymer selected from the group consisting of polyepichlorohydrin, epichlorohydrin rubber, epichlorohydrin-ethylene oxide copolymers or terpolymers of epichlorhydrin and the tertiary amine contains at least two tertiary amino groups.
 9. Ion conducting glue for gluing exchange membranes comprising exchanging molded bodies of the type produced from a casting solution ofpolymers containing reactive halogen-containing functional groups; and tertiary aminesby evaporation of the solvent or phase inversion wherein the polymer which contains reactive halogen-containing functional groups is an epichlorohydrin polymer selected from the group consisting of polyepichlorohydrin, epichlorohydrin rubber, epichlorohydrin-ethylene oxide copolymers or terpolymers of epichlorhydrin and the tertiary amine contains at least two tertiary amino groups. 